The only cards I would consider watercooling is something like the 4850x2 or 4870x2 since the stock coolers are loud and they aren't compatible with air cooled heatsinks.

I've water cooled all my video cards, past and present, it makes a huge difference in temps and noise levels.......................

yeah, but I still don't think it makes sense when the water cooling setup would cost more than his current graphics cards. For the cost of a rad, pump, tubing, fans, and GPU block he could get a high end card that would be much faster than the best overclocks he could get off his 8400 gs.